1. Field of the Invention
The invention relates generally to low friction coatings formed on load bearing surfaces that slidingly engage each other, such as a gate and seat ring of a gate valve for a wellhead assembly.
2. Background of the Invention
Gate valves are used when a straight-line flow of fluid and minimum flow restriction are required. When the valve is wide open, the gate is drawn into the opposite end of the valve cavity. The gate has an opening for flow through the valve the same size as the pipe in which the valve is installed. The valve provides an unobstructed passageway when fully open. It is best suited for main fluid supply lines and for pump lines, and is often used for oil and gas production where pressures may range from 5000 to 30,000 psi.
Previous versions of gate valves have featured a coating on the exterior surface of the valve's gate and seats for reducing friction, as well as to reduce corrosion and improve wear resistance. Some previous versions have utilized layers of hard facing, such as tungsten carbide, upon the surface of the valve's gate and seats. Other previous versions have utilized a vapor deposition process or a chemical vapor deposition to coat the exterior surface of the valve's gate and seats.
Prior art gate valves rely on liquid lubrication to minimize the adhesive forces between these materials. Liquid lubricants, such as hydrocarbon and silicone based greases, decrease in both viscosity and surface tension as their temperature is increased, thereby minimizing the protective boundary layer they offer to the highly loaded surfaces. Additionally, only very expensive greases are stable to temperatures above 400 F. and may lose some of their mass and lubricating properties. The loss of lubrication at high temperatures leads to significant increases in valve torques and may lead to the galling of the mating surfaces.
Polymer coatings have been used on sliding load bearing surfaces in general, including on ball valves. Some polymer type coatings have been used on gate valves as well, but suffer from insufficient load bearing capacity and ductility especially at elevated temperatures. A thermoplastic polymer coating tends to creep and flow under high contact stress and elevated temperatures. A thermoset type of polymer coating does not soften with temperature as does a thermoplastic, but suffers from poor ductility and a propensity toward greater adhesion especially at elevated temperatures. These properties generally result in cracks in the coating and the removal of the coating to its mated surface.